Understanding Rate Effects in Cohesive Soils

Military infrastructure composed of geomaterials and soils have been extremely commonplace in the construction of defensive structures to protect vital personnel and assets. The necessity for soils to withstand extreme loading from the effects of blast and fragmentation prompts the understanding of soil behaviour within these loading regimes to be of great significance, especially to enable the development of more rigorous soil models and proficient methods for design engineers to counteract new threats.

While the use of sand and gravel have been well researched in current literature, the behaviour of cohesive soils such as clays as a protective material against high-strain loading is an under explored area of research. Subsequently, a primary property of soils is the strength exhibited when the material is affected by a lateral confining pressure. The response of a material under particular confining mechanisms when subject to blast or fragmentation is a key area of investigation. While this is commonplace in standard geotechnical engineering testing at lower stresses and strains, it becomes considerably more challenging at higher stresses and strain rates

One-dimensional compression tests are conducted using the split-Hopkinson pressure bar (SHPB) to investigate strain rate dependence and strength of the soil. Conventional free field SHPB experiments typically used with solid materials is compared with rigid lateral confinement mechanisms traditionally used on soft materials, this provides a basis of extremity and prompts a foundation for the application of a modified SHPB setup that allows for the soil sample to be partial laterally confined within a water reservoir during SHPB tests. This modified SHPB apparatus allows for lateral stresses to be monitored by measuring the change in pressure in the water reservoir through the use of a pressure transducer, yet the test still proceeds in a similar manner to an unconfined case and hence allows for triaxial behaviour to be much better characterised.

Modern numerical modelling techniques to capture soil behaviour and high-strain-rate testing are evaluated using LS-DYNA. The performance of various material models and geometrical techniques was assessed to determine the effectiveness and limitations with current numerical modelling techniques. The development of numerical models are coupled with experimental data to provide a comprehensive characterisation of cohesive soils and the factors that affect their attenuation of high-strain-rate loading such as blast or fragmentation.